The United States is expected to help bolster the new British-Irish agreement for governing Northern Ireland with a U.S. aid program that some congressional sources described yesterday as a "mini-Marshall Plan" that could range in value from $250 million to $1 billion.

State Department officials and other congressional sources, noting that discussions between the administration and Congress on an aid package are just beginning, cautioned that it is too early to make any realistic predictions about its size and scope.

But these sources agreed that there appears to be a broad consensus that the United States should help to ensure successful implementation of the historic agreement signed last Friday by Prime Ministers Margaret Thatcher of Britain and Garret FitzGerald of Ireland.

The accord would give Dublin an official voice in governing Northern Ireland, which has been under British rule since the island's partition 64 years ago.

The aim is to end 16 years of sectarian violence between the majority Protestant and minority Roman Catholic communities in Ulster, as the province historically is known, by providing better protection for the Catholics' rights and interests.

As a result, the sources continued, the administration and Congress seem certain to agree by next spring on an aid package aimed at stimulating Northern Ireland's troubled economy through direct financial assistance and incentives for U.S. firms to invest there.

However, the sources stressed, several problems must be resolved on both sides of the Atlantic before a rational aid plan can evolve. First, they noted, the agreement must be approved by the British and Irish parliaments and the two governments must devise a plan for equitable distribution and effective use of any U.S. aid.

In this country, the sources added, the desire to provide help must be reconciled with pressures for budgetary austerity generated by demands for reducing the deficit.

Some sources, pointing to the voting strength of the Irish-American community and its emotional support of Ulster Catholics, said Congress might be inclined to push for an aid package larger than what President Reagan deems prudent.

In hailing the agreement last Friday, Reagan said he hoped that "we can help . . . in restoring sound economics there." But his statement appeared to emphasize private investment, while other expressions of support from key members of Congress spoke more explicitly of "financial and economic support."

The sources said preliminary talk on Capitol Hill has included discussion of programs involving $1 billion. But they acknowledged that Congress and the administration are unlikely to agree on such sums.

Instead, they said, as the specifics of an aid package begin to take shape, its price tag is much more likely to approximate the lower range of $250 million or less.

However, while the numbers are subject to debate, there seems to be no question about the depth and breadth of congressional support for the principle of aid to Ulster.

Within hours after Friday's agreement was announced, Sen. Edward M. Kennedy (D-Mass.) was joined by 21 other senators in a pledge to consider aid, and in the House, Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.), had obtained similar commitments from 31 members with influence over foreign policy and appropriations matters.